1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, for example, a resonator or a filter, and more particularly, to a surface acoustic wave device having an asymmetrical double electrode used as a unidirectional interdigital transducer or a dispersive reflection type reflector.
2. Description of the Related Art
A surface acoustic wave device such as a surface acoustic wave filter is widely used in mobile communication equipment or broadcasting equipment, or other such apparatuses. Particularly because the surface acoustic wave device is compact, lightweight, tuning-free and easy to manufacture, the surface acoustic wave device is suitable for an electronic component for use in portable communication equipment.
The surface acoustic wave device is broadly divided into a transversal type filter and a resonator-type filter, based on its structure. In general, the transversal type filter has advantages of having (1) a small group delay deviation, (2) a superior phase linearity, and (3) a high degree of flexibility in the pass band design based on weighting. However, the transversal type filter has a disadvantage of having a large insertion loss.
An interdigital transducer (hereinafter referred to as an “IDT”) used in a surface acoustic wave filter transmits and receives surface acoustic waves with respect to both sides of an IDT, that is, the IDT transmits and receives surface acoustic waves bilaterally in an equal manner. For example, in a transversal type filter in which two IDTs are spaced apart from each other by a predetermined distance, one half of the surface acoustic waves transmitted from one IDT is received by the other IDT, but the surface acoustic waves propagated from the one IDT to the opposite side of the other IDT become a loss. This loss is called a “two-way loss”, and has become a big factor in increasing insertion loss of a transversal type filter.
In order to reduce the above-described two-way loss, various types of unidirectional IDTs have been proposed. In such unidirectional IDTs, surface acoustic waves are transmitted and received at only one side alone thereof. Also, low-loss transversal type filters which utilize these unidirectional IDTs have been developed.
For example, Hanma et al., have proposed an asymmetrical double electrode in “A TRIPLE TRANSIT SUPPRESSION TECHNIQUE” 1976 IEEE Ultrasonics Symposium Proceedings pp. 328-331. FIG. 14 is a schematic partially cutaway plan view showing the asymmetrical double electrode disclosed in this prior art.
In an asymmetrical double electrode 101, half wavelength sections Z constituted of two strips 102 and 103 having different widths from each other, are disposed repeatedly many times along the propagation direction of surface acoustic waves. Such an electrode defined by half wavelength sections Z constituted of two strips having different widths from each other, is called an “unbalanced double electrode” or a “asymmetrical double electrode”.
The width of a half wavelength section is set to 0.5λ. The width of a strip 102 having a relatively narrow width is set to λ/16. The width of a strip 103 having a relatively wide width is set to 3λ/16. The width of a gap between the strips 102 and 103 is set to 2λ/16. The width of an outer gap of the strip 102 in the half wavelength section is set to λ/16. The width of the outer gap of the strip 103 in the propagation direction of surface acoustic waves in the half wavelength section is set to λ/16.
Between adjacent basic sections, the electrical polarities are opposite to each other.
In the above-described asymmetrical double electrode, a reflection per basic section can be expressed by a resultant vector that is generated by synthesizing reflected waves from the edges X1 to X4 of the strips 102 and 103 shown in FIG. 15. FIG. 16 shows the reflection vectors at the edges X1 to X4 when the reference position is set to the center of a basic section, and the resultant vector thereof. As can be seen from FIG. 16, the resultant vector V is located at an angle of 67.5°, and the reflection center is located at an angle of 67.5°/2=33.75°.
Also, in this asymmetrical double electrode, the outer edge X1 of the strip 102 and the outer edge X4 of the strip 103 are disposed bilaterally symmetrically with respect to the center of the half wavelength section. Hence, the distances between the center of a basic section and the outer edges of the nearest strips in the adjacent basic sections, are also equal to each other. In the asymmetrical double electrode, therefore, an excitation center is located at the center of the basic section Z, with a phase difference of about 33.75° generated between an excitation center and the reflection center. Thus, the asymmetrical double electrode operates as a unidirectional electrode.
Table 1 below shows the inter-mode coupling coefficient κ12/k0, the phase difference between the excitation center ψ and the reflection center φ, and the reflection center φ, when forming an asymmetrical double electrode of aluminum film having a 3% film-thickness on a ST-cut crystal quartz substrate, as an example of the above-described asymmetrical double electrode.
TABLE 1ItemCalculated valueInter-mode coupling coefficient κ12/k0 0.00257Phase difference between excitation center ψ31.3°and reflection center φReflection center φ33.8°
Here, k0 is a wave number of surface acoustic waves propagating through an IDT. The ratio κ12/k0 and the phase difference between the excitation center ψ and the reflection center φ can be obtained from the resonant frequency determined by the finite element method, using the technique of Obuchi et al., (“Evaluation of Excitation Characteristics of Surface Acoustic Wave Interdigital Electrode Based on Mode Coupling Theory”, Institute of Electronics, Information and Communication Engineers of Japan, Technical Report MW90-62). Also, the reflection center φ is determined by the phase difference between the excitation center ψ and the reflection center φ, and the excitation center obtained from the fundamental wave component which is acquired by Fourier-transforming the electric charge density distribution on the electrode obtained by the finite element method.
Japanese Unexamined Patent Application Publication No. 61-6917 discloses an electrode which has implemented unidirectionality by disposing two strips having mutually different widths in a half wavelength section, as in the case of the above-described asymmetrical double electrode. The electrode disclosed in this Japanese Unexamined Patent Application Publication No. 61-6917 is also supposed to operate as a unidirectional electrode due to the asymmetry of the two strips thereof. However, in the method disclosed in the Japanese Unexamined Patent Application Publication No. 61-6917, no means for controlling the reflection center and the reflection amount are disclosed. In addition, no feasible reflection center and reflection amount are described.
The article “Direct Numeral Analysis SAW Mode Coupling Equation and Applications Thereof”, 27th EM symposium preprint, pp. 109-116, Takeuchi et al., describes the principle of a unidirectional IDT which provides flat directivity over a wide band in the structure wherein positive and negative reflection elements are dispersively disposed in a unidirectional IDT. Herein, however, no means for forming a reliably superior unidirectional IDT are described.
In general, when surface acoustic waves are caused to be incident on an IDT constituted only of double strips without reflection, reflection is caused by re-excitation. As a result, in the case of a conventional transversal type filter, waves called “triple transit echo” or TTE, occur, and cause ripples or other undesired wave characteristics that adversely effect filter characteristics. The above-described literature by Hanma et al., discloses a method for canceling out reflection due to re-excitation by means of acoustic reflected waves of an asymmetrical double electrode. This method, however, has created a problem that new ripples are caused by acoustic reflection when the acoustic reflection is larger than the reflection caused by the re-excitation. Therefore, such a method for canceling out the reflection by re-excitation is subjected to the restriction of piezoelectric substrate material or electrode material, since the reflection vector length which represents the acoustic reflection amount is fixed in an asymmetrical double electrode.
On the other hand, the article “About One Weighting Method For SAW Reflector”, 1999, General Convention of Institute of Electronics, Information and Communication Engineers of Japan, p. 279, Tajima et al., discloses a method for performing weighting with respect to the reflection coefficient of a reflector. This method uses a plurality of strips having mutually different widths and makes use of the change of the reflection coefficient of a strip based on the strip width. However, when the strip width is changed, the sonic speed is also changed. As a result, when attempting to perform weighting based on the strip width, a testing method and apparatus is needed to find a correct sonic speed and to change the arrangement pitch of the strip in accordance with this corrected sonic speed. This poses a problem that the design requires an extremely high degree of technique.
As described above, various IDTs or resonators each operating as a unidirectional electrode by asymmetry of two strips have been proposed, but conventional asymmetrical double electrodes have not yet achieved sufficient unidirectionality. In addition, the reflection center and the reflection amount of the conventional asymmetrical double electrodes have been very difficult to control.